The Germans learn these cases in the school but then they forget. They just know how to make correct sentences. Every language has its unique rules. Why has Chinese tone system? We cannot ask, why? We have to accept the rules of any language.
The problem with foreigners is that they are taught in a wrong way. They are just fed with the rules of Grammar and they keep thinking in terms of Grammar, der die das, deklination, endings, accusative, dative, .... etc. So they cannot speak the language fluently.
The best way to learn is to remember standard expressions and sentences without thinking about the grammar. These standard sentences will later serve as a reference when something is unclear.
For example learn the expressions like, "in manchen Fällen .. ", "in anderen Ländern.." , "mit einigen Beispielen"..... etc. instead of learning the dative plural rule with ending.